Additive manufacturing has become a more and more attractive solution for the manufacturing of metallic or ceramic functional prototypes and components. For example, SLM and SEBM (selective electron beam melting) methods use powder material as base material. The powder is molten by laser or electron beam. According to a CAD model there is a powder layer deposition and the desired object is built on a platform. That means the component or article is generated directly from a powder bed by a layer wise manufacturing.
Conventional scanning strategies, as they are applied in all SLM machines nowadays, use centro-symmetrical laser spot configurations providing uniform irradiation conditions in all scanning directions. Melting of powder material is realized by (often at least partially overlapping) parallel laser passes, so-called tracks. During the additive manufacturing process, powder in the selected area where subjected to laser radiation is molten into the desired cross section of the part.
On one side, the production rate of the process is determined by the powder melting parameters: Power of the energy beam, scanning speed, hatch distance, spot size. To ensure, that powder will be irradiated with an equal amount of laser radiation in its entire cross-section, a small diameter laser beam is traversing the entire cross section in multiple parallel and usually equidistant trajectories. In general, the distance between consecutive laser passes is kept constant during the entire manufacturing process. This so-called hatch distance is defined by the laser spot size at the powder bed and by complex laser-powder interaction phenomena. For maximum productivity it is normally attempted to make best use of the available power of the energy beam.
Another important parameter is the scanning speed. It can be kept constant or variable along the scanning trajectory. These three key process parameters are mutually connected and cannot be changed arbitrarily. For example, undesired porosity appears, if the laser energy density at the powder bed is reduced too much, e.g. due to insufficient available laser power at the powder bed or too high scanning rates.
On the other side, the production rate depends significantly on secondary ‘laser idle’ operations, where the energy beam is switched off. Examples for such ‘laser idle’ steps are repositioning jumps, beam acceleration and deceleration before and after beam repositioning, or “jump”, “mark” and “poly” delays, associated with movement commands for the energy beam. In particular for complex thin walled parts, where the remelting of the cross section involves a large number of short tracks and many repositioning movements of the energy beam, these delays accumulate to a significant fraction of the overall manufacturing time. Secondary ‘laser idle’ time also includes the application of new powder layers with its associated wait times.
For example, an SLM machine consists in general of a control unit, a laser and a laser beam focusing and optical beam shaping system, an isolated work chamber with protective atmosphere, a mechanical system for powder deposition from a sealed container to the work zone. The optical laser beam shaping and beam guiding system typically consists of a beam expander, scanner head and a focusing lens. The scanner head is responsible for the laser beam positioning at selected cross-sections of the powder bed. It normally consists of two mirrors attached to two electric motors to aim the incident laser beam at the desired direction. It is desirable to tune the process parameters in a way to keep ‘laser idle’ time as low as possible, but it cannot be reduced to zero.
The key process step in SLM is the selective laser melting of the powder layers.
FIG. 1 shows a typical conventional algorithm of irradiating a cross-section 1 by laser beam known from the prior art. Several laser tracks 2 are equally separated from each other by a hatch distance 4. The track length 3 of the laser can be smaller, equal or larger than the width w of cross-section 1. In FIG. 1 the track length 3 is equal to the width w. FIG. 1a) and FIG. 1c) show typical unidirectional scanner paths, FIG. 1b) and FIG. 1d) show bidirectional scanner processing.
Such conventional scanning strategies are realized in the following way: The laser beam is positioned at the start (acceleration) zone 6 of the irradiation trajectory 2 (see FIG. 1c) and FIG. 1d)), the movement of the laser starts and the system switches the laser on. The focused laser beam is translated along the scanning direction on the irradiation trajectory 2. At the end of the irradiation trajectory 2, the laser is switched off, decelerated (see dotted line 7, deceleration zone) and it is then repositioned (see dotted line 8, reposition movement) to the following start point of the next laser track 2 in the linear translation direction 5. In some cases, the acceleration and deceleration zones 6, resp. 7 are made very short, or they are completely eliminated. In FIG. 1a) and FIG. 1c) adjacent laser tracks 2 are done in the same direction, in FIG. 1c) and FIG. 1d) adjacent laser tracks 2 are done in opposite directions.
It can be clearly seen, that the irradiation process consists of the selective laser melting step (shown by the arrows 2) and secondary ‘laser idle’ operation steps (shown by the dotted arrows 6, 7 and 8).
The later ones are one explanation for a major obstacle for the commercial success of those additive manufacturing processes—the low build rate.
This low build rate leads to long process times with associated high manufacturing costs and only low to moderate throughput for a serial production. For example, the manufacturing cycle of a single component of a few kg of weight can take 100 machine hours and more.
Various solutions are known to increase the production rate. Among them is the application of multi-beam technology, preheating of manufacturing area to the temperatures around 500-900° C., or the modification of the intensity distribution of laser beam.
Multi-beam technology uses multiple independent laser beams to melt the powder. Indeed, this leads to increased production rates, but it requires at the same time equivalent amount of additional investment costs.
Preheating of the powder bed is a method to decrease the amount of energy required to melt the powder material. However, the application of such methods requires additional investment costs, caused by the much more complex hardware requirements. It is also complicated by undesired secondary effects, like powder sintering in the work zone.
The application of a non-gaussian laser power density distribution in the focal plane can improve the SLM process. The negative aspect of this approach is the significant initial investments required for its application.
Document US 2012/267347 A1 describes a method for welding workpieces made of highly heat-resistant superalloys, including a particular mass feed rate of the welding filler material. The disclosed prior art method is a laser metal forming (LMF) process, which is a blown powder technique and which is used for refurbishment of damaged regions in order to restore the original wall thickness. The method described in Document US 2012/267347 A1 is therefore not a selective laser melting (SLM) process which generates new articles or parts of articles layer by layer directly from a powder bed. The method according to US 2012/267347 A1 includes generating a heat input zone on a workpiece surface by means of a heat source, feeding welding filler material into that zone by means of a feeding device, and generating a relative movement between the heat source and the feeding device on one hand and the workpiece surface on the other hand by means of a conveying device. This has the disadvantage that heavy parts, like CNC table, robot arms or laser head have to be moved. The welding pattern according to this prior art document are meandering weld lines with rounded edges.